1. Field of the Invention
This invention relates generally to the field of adhesives, more particularly to adhesives useful in the area of forest products, such as plywood, chipboard, waferboard, strandboard, etc. Methods of synthesis, the resulting adhesive and methods of using the adhesive are within the scope of the invention.
2. Description of the Related Art
Forest products, especially products which are manufactured into useful materials through the use of adhesives are generally known. Chipboard, waferboard, strandboard, plywood and other composites made through the addition of an adhesive to forest products or byproducts is well established. Generally, the adhesive comprises a phenol-formaldehyde (PF) resin. Another commonly used resin is a liquid polymeric 4,4xe2x80x2-methylene-bis(phenyl isocyanate) (pMDI). Although aqueous, alkaline phenol-formaldehyde resins show good durability, relatively low cost and relatively low toxicity, they are known to exhibit slower press times and, in general, produce products with higher thickness swell properties than the same products composed of a pMDI binder. Although isocyanate resins can exhibit some enhanced performance, they are more costly than PF resins.
An additional drawback to production in manufacturing plants is the huge capital costs associated with the press and associated equipment, including steam generation equipment which provides the heat during the pressing process. Yet the press itself is a bottleneck in the process because of the dwell time of the product required in the press in order to cure the adhesive. Thus, any manner of reducing dwell time would be of commercial importance to the board making industry.
Thus, there exists a need for suitable adhesive compositions for the manufacture of improved wood products, especially exterior grade products, such as waferboard and oriented strandboard.
It is, therefore, an object of the invention to provide novel adhesive systems which avoid the problems associated with the known adhesives.
It is a further object of the invention to provide a hybrid resin comprising the combination of a PF resin and pMDI that forms a single phase liquid material, and which has both shelf stability, and fast cure times when used in conventional board making processes.
We have found that by combining a protected PF resin and pMDI we can obtain a storage life of greater than 2 weeks at room temperature.
The hybrid resin of the invention can be applied to forest products by applying, e.g., by spraying, blending or otherwise mixing the adhesive and lignocelluosic material, such as wood flakes, wood fibers, wood particles, wood wafers, strips or strands, or other comminuted lignocellulosic materials while the materials are tumbled or agitated in a blender or similar apparatus. Once blended, the materials are formed into a loose mat which, optionally after orientation of the lignocellulosic materials, is compressed between heated platens or plates to set the binder and bond the flakes, strands, strips, pieces, etc. together in densified form.
Conventional processes are carried out at elevated temperatures of from about 120 to 225xc2x0 C., by using a source of heat, such as steam, to heat the platens, or even to inject the steam into the mat, to cure the resin.
Alternatively, the blended material may be fed to molds for the purpose of forming molded articles in which the resin and particles are bonded under heat and pressure. However, notwithstanding the particular shaping process employed, the resin of the invention has a faster cure time than any of its components.
The adhesive of the invention has other utilities, such as being coated upon veneers or strips of wood, laminates, etc. by roll coating, knife coating, curtain coating or spraying the adhesive onto the veneer surface(s). A plurality of veneers are then laid-up to form sheets of the required thickness and subjected to heat and pressure to effect consolidation and curing of the materials into a board.
Synthesis of the novel adhesives of the invention are also disclosed in more detail in connection with the description of the preferred embodiments.
Phenol-formaldehyde resins are generally well known to fall into two classes depending upon the phenol to formaldehyde ratio and being generally termed xe2x80x9cnovolacsxe2x80x9d or xe2x80x9cnovolaksxe2x80x9d (which are thermoplastic) and xe2x80x9cresolesxe2x80x9d (which are thermosetting).
On the other hand, pMDI resins used to bond cellulosic materials are also reported in the prior art; See, U.S. Pat. Nos. 3,666,953; 5,008,359; 5,140,086; 5,143,768; and 5,204,176, the entire disclosures of which are herein incorporated by reference.
To mediate the undesirable properties of PF and pMDI, the inventors thought to combine the two resin systems. However, the physical combination of the two binders is problematic because of differences in polarity and incompatible chemical functional groups. A strategy to produce a hybrid resin by emulsification of the pMDI in PF resin and chemical modification of the NCO functional group of the pMDI proved not to give a satisfactory combination of storage life and performance. Thus, the inventors conceived of the invention by modifying the PF resin.
The inventors protected the functional group of the PF resin, e.g., by acylating the PF resin, and found that the esterfication of the phenolic oxygen greatly diminished the reactivity of the PF portion of the mixture when combined with pMDI to form a single phase liquid material with a storage life of greater than 2 weeks at room temperature.
The diminished reactivity of the PF portion is well illustrated by its gel time in excess of 350 hours at 125xc2x0 C. The gel time of the pMDI alone, or with added water to mimic the conditions needed to cure the hybrid resin is in excess of 100 min. at 121xc2x0 C. However the gel time with added water of the acylated PF/pMDI system is about 10 min.
The acyl PF resin can be produced by any known method of hydroxyl group acylation, to yield an organic-soluble anhydrous clear liquid. The acylated PF resin can than be added to any commercially available pMDI resin, or a pMDI resin synthesized to customer""s specifications.
When the acyl group is a carboxylic ester, it acts as a protecting group that permits reactivity of the hydroxyl oxygens with the NCO functionality of the pMDI resin. Under conditions of temperature and moisture encountered during manufacture of boards, such as strandboard, the PF portion is deprotected and the PF and pMDI can react with one another.
There is a wide combination of phenolic resins and acyl groups that can be employed to render the PF resin inactive toward reaction with isocyanate groups as disclosed in U.S. Pat. Nos. 5,051,454 and 5,340,888, the entire disclosures of which are herein incorporated by reference.
Suitable acyl groups include acetate, such as acetoacetate, and groups such as ketene and diketene groups may be used to form the acyl protecting groups. Additionally, groups such as formate, acrylate, propionate, lactate, crotonate, methacrylate, butyrate, isobutyrate, caproate, caprylate, benzoate, toluate, p-amino-benzoate, p-hydroxybenzoate, salicylate, cinnamate, laurate, myristate, palmitate, oleate, ricinoleate, stearate, oxalate, euccinate, fumarate, maleate, adipate, phthalate, azelate and sebacate may be used to form acyl protecting groups in accordance with the invention.
Because of the inherent protecting nature of the acyl groups, the acyl PF resin can be used in combination with any pMDI resin that is useful in commercial panel production.
We have found that the two resins may be combined in amounts of from 10-80 wt % PF resin based on the weight of the whole system. However, we have also found that hybrid resins comprised of 10-35 wt % PF exhibit better lack of viscosity advancement over a period of four weeks. With the addition of water in acetone, a 30 wt % PF/pMDI hybrid resin will react at 121xc2x0 C. to form a gel, which hardens to form a single phase material. We have also found that strandboard panels produced with a 40 wt % PF/60 wt % pMDI hybrid resin exhibit cure speeds that appear much faster than either of its two components above. This synergistic effect was not expected by us.